In this paper, to get a deeper insight on the fs laser nanostructuring of materials, the authors suggest a numerical model describing the propagation and energy deposition of a single fs laser pulse in bulk silicon, silicon on insulator, and gold thin film under laser ablation conditions. A brief description of the authors’ strategy is as follows: their numerical modeling starts by using a finite difference time domain (FDTD) method to solve Maxwell’s equations. For the first optical half-cycle, they assume the laser pulse propagates in an unexcited material. From the next optical half-cycle until the end of the pulse, at every optical half-cycle the authors keep the optical susceptibilities of materials updated by calculating the carrier and/or heat diffusion equations with various source terms depending on the intensity of light. For silicon samples, the optical susceptibility changes with the number of carriers in the conduction band, increased by various routes during optical excitation. Therefore, the authors calculate the carrier diffusion equation by considering the source terms including one-photon absorption, two-photon absorption, and impact ionization, and also solve the heat equation to take into account the effective mass and diffusivity, both as functions of carrier temperature, to estimate the susceptibility. In the case of gold, the authors only consider the heat equation to use the electron-electron collision frequency as a function of electron temperature retrieved from experiments, and then update the susceptibility of gold at every optical half-cycle. Using this strategy, the authors successfully show that the self-reflectivity of silicon and gold samples under ablation conditions is consistent with experiments. Moreover, they are also able to pick out the dominant mechanisms causing the change in the self-reflectivity of samples with the fluence of laser. Therefore, not only can this method be extended to further understand the heat and charge transport after the excitation as the authors note, but it can also be a very useful tool to find a better condition for advanced laser processing.
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